So NPR has a list of the Top 100 fantasy and SF novels, as selected by whomever showed up. Imagine if they made a list of the best BBQ places in the USA. You know, you know, for a fact you know, that the McRib would be on the list. Only an idiot thinks the McRib would belong on such a list, but there it would be, displacing some worthier BBQ source.
So this list has, along with a lot of obvious choices, a fair number of McRibs. Terry Brook's Shannara books? Really? There's no questioning their import to Fantasy Inc.'s history of homogenizing and pasteurizing Tolkien/Robert "Conan" Howard into reproducible comfort food, prefab daydreams, but what's it doing on a list that aspires to quality? And Piers Anthony? Eek. I like what I've read by George R. R. Martin, but would his work rank so high if he didn't have a breakout hit TV show? No, it would not. I like Neil Gaiman, but he's been the flavor of the month in fantasy circles since the 90s. That's a long month. Good soul that he is, I'm sure he'd be happy to bump one or two of his titles from the list to make room for Mervyn Peake, Elizabeth Hand, Robert Holdstock.
As far as fantasy is concerned, you're better off going by Ballantine Adult Fantasy. Obviously it's a bit dated, so titles like Viriconium and Little, Big aren't there, but they aren't on NPR's list either.
As for SF, seek out a copy (a local library surely has one) of John Clute's Science Fiction: The Illustrated Encyclopedia. It's about as navigable and profusely illustrated as a good magazine or webpage, with substantial writeups on numerous worthy texts and authors, many of whom were not included on NPR's list.
In closing, here's a silly faux-trailer for an overrated Mary-Sue fantasy novel that happily didn't make the list: War For the Oaks by Emma Bull.
I like the atmospheric opening, but the Faerie Court is pure Renfaire.